google: Make sure time is always in UTC

If times are stored in different time zones, then we occasionally get heisenbugs about expired tokens

Change-Id: I0c117977688d8d6c7b12b211092e5040a41a1f46
GitHub-Last-Rev: 3ff51b34f58e0bde7946af59e1215b7d3db65a95
GitHub-Pull-Request: golang/oauth2#482
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/300929
Reviewed-by: Tyler Bui-Palsulich <tbp@google.com>
Trust: Tyler Bui-Palsulich <tbp@google.com>
Trust: Cody Oss <codyoss@google.com>
Run-TryBot: Tyler Bui-Palsulich <tbp@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
diff --git a/google/internal/externalaccount/basecredentials.go b/google/internal/externalaccount/basecredentials.go
index 1b87c09..1a6e93c 100644
--- a/google/internal/externalaccount/basecredentials.go
+++ b/google/internal/externalaccount/basecredentials.go
@@ -14,7 +14,9 @@
 )
 
 // now aliases time.Now for testing
-var now = time.Now
+var now = func() time.Time {
+	return time.Now().UTC()
+}
 
 // Config stores the configuration for fetching tokens with external credentials.
 type Config struct {